Device for producing methane gas and use of such a device

ABSTRACT

The invention relates to a device for producing methane gas including a chamber, means for conveying the product in the chamber which comprise a screw mounted to turn in the chamber along a geometric axis of rotation, Joule-effect heating means of the screw, a removal unit for removing impurities present in the gas coming from the thermal treatment of the product, said unit being connected to a high outlet of the chamber, and a purification system for purifying the gas at the outlet of the removal unit.

The invention relates to a device for producing a methane gas, i.e. a gas having methane as its main component. The invention also relates to the use of such a device for recycling a substance of the solid recovered fuel (SRF) type or of the polymer material type.

TECHNOLOGICAL BACKGROUND OF THE INVENTION

Although valued for its energy and environmental properties, natural gas presents the drawback of being a non-renewable commodity and global supplies are being quickly depleted.

Since natural gas is on average made up of 95% methane, it has been envisaged to take advantage of the natural phenomenon of methanization of organic waste that produces a substantial amount of methane in order to create industrial gas, generally referred to as biogas, so as to offer an alternative to natural gas. Furthermore, this makes it possible to recycle waste of the biomass type.

Conventionally, gasification steps are used to force methanization of the biomass and to extract methane therefrom.

However, such steps do not make it possible to recover gas that is very rich in methane. The gas at the outlet thus typically contains 2% to 6% methane, which is very far from the 95% rate of natural gas.

OBJECT OF THE INVENTION

An object of the invention is to provide a device making it possible to generate a methane gas as well as the use of said device for recycling substances of the SRF type or of the polymer material type.

BRIEF DESCRIPTION OF THE INVENTION

In order to achieve this object, the invention provides a device for producing a methane gas by heat-treating a substance in the form of divided solids, the device comprising:

an enclosure comprising a substance feed inlet, a bottom outlet for recovering residues of the treated substance, and a top outlet for extracting the gas coming from the treatment of the substance;

conveyor means for conveying the substance between an inlet of the enclosure and the bottom outlet, the means comprising a screw mounted to rotate inside the enclosure about an axis of rotation and having drive means for driving the screw in rotation;

heater means for heating the screw by the Joule effect;

an impurity elimination unit for eliminating impurities present in the gas coming from the heat treatment substance, said unit being connected to the top outlet of the enclosure; and

a purification system for purifying the gas at the outlet of the elimination unit, the purification system being connected to the elimination unit.

Thus, the substance is introduced into the inlet of the enclosure in the form of divided solids and the screw pushes the divided solids in continuous manner towards the bottom outlet of the enclosure. Due to the advantageous way the screw is heated by the Joule effect, the divided solids are heated very quickly and they transform without adhering to the turns of the screw, thus generating a gas already presenting a high methane content (greater than 50% when the substance is of the SRF type or of the polymer material type such as plastics). Subsequently associating two consecutive gas treatment units makes it possible initially to remove impurities of the dust, solid particle, tar, oil, . . . , type from the gas, and secondly to purify the gas by separating the methane from the other gaseous components.

At the outlet of the invention, there is thus obtained a gas that is very rich in methane. Experiments performed by the Applicant have thus made it possible to obtain methane content of substantially 92%.

The invention thus makes it possible to obtain a gas having a methane concentration that is close to that of a natural gas. The gas obtained by the invention is therefore a very good alternative to natural gas.

In addition, the gas at the outlet of the invention can be injected directly into containers (cylinders, tanks . . . ) or into a gas distribution network.

Furthermore, the invention may be supplied with all kinds of substances such as biomass, but is particularly advantageous with substances of the SRF type or of the polymer material type such as plastics. This is particularly advantageous in a context of ever-increasing recycling of waste, in particular non-fermentable waste for which recycling is less developed.

In the present invention, the impurity elimination unit for eliminating impurities that are undesirable and polluting in general manner is thus distinguished from the purification system, which does not serve to clean the gas, but which enriches it with methane by separating it from other gas.

In the meaning of the invention, the term “methane gas” refers to a gas having methane as its main component, it being understood that said gas may contain other components such as dinitrogen in smaller proportions.

In another particular aspect of the invention, the device includes an inlet chimney that is connected to the inlet of the enclosure and that comprises leaktight connection means for connection to the inlet of the enclosure so as to limit the air entering the enclosure.

In another particular aspect of the invention, the device includes an outlet chimney that is connected to the bottom outlet of the enclosure and that comprises leaktight connection means for connection to the bottom outlet of the enclosure so as to limit the air entering the enclosure.

In a particular aspect of the invention, the impurity elimination unit comprises cracking means for cracking the gas.

In a particular aspect of the invention, the impurity elimination unit comprises condensation means for condensing polluting condensates of the gas.

In a particular aspect of the invention, the impurity elimination unit comprises filter means for filtering dust and solid particles present in the gas.

In a particular aspect of the invention, the filter means are connected to the outlet of the cracking means or to the outlet of the condensation means.

In a particular aspect of the invention, the filter means comprise a high-temperature cyclone or a high-temperature ceramic filter.

In a particular aspect of the invention, the purification system comprises at least two distinct purification stages.

In a particular aspect of the invention, the purification system comprises at least three distinct purification stages.

In a particular aspect of the invention, the purification system comprises condensation means for condensing condensable phases of the gas at a pressure that is less than atmospheric pressure measured at sea level.

In a particular aspect of the invention, the condensation means are shaped so as to ensure separation of the gas from the condensable phases at a pressure lying in the range 0.04 bar to 0.3 bar.

In a particular aspect of the invention, the purification system comprises pressure swing adsorption equipment.

In a particular aspect of the invention, the pressure swing adsorption equipment is connected to the outlet of the condensation means.

In a particular aspect of the invention, the purification system comprises a filtering machine.

In a particular aspect of the invention, the filtering machine is a membrane separation machine.

In a particular aspect of the invention, the filtering machine is connected to the outlet of the pressure swing adsorption equipment.

In addition, the invention relates to the use of an above-mentioned device for recycling a substance of the SRF type or of the polymer material type.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be better understood in light of the description given below with reference to the sole figure diagrammatically showing the device in a particular non-limiting embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the sole figure, the device in a particular embodiment of the invention makes it possible to produce a methane gas by applying heat-treatment, in this embodiment pyrolysis, to a substance in the form of divided solids.

By way of example, the substance is formed of polymer matter. The substance is typically made of plastics comprising for the most part polyethylene and polyethylene terephthalate. In particular manner, the divided solids are present in the form of three-dimensional granular solids of the granule or pellet type. The maximum dimensions of said divided solids preferably lie in the range 2 millimeters (mm) to 30 mm.

The device of the invention comprises an enclosure 1 extending essentially in a horizontal direction and held at a distance from the ground by legs (not shown). The enclosure 1 comprises an outer casing, in a single piece in this example, e.g. made of metal, and in particular made of non-magnetic stainless steel. In a particular embodiment, the enclosure 1 also includes an inner casing made as a single piece of refractory material. Respective equipment boxes 3 are attached to each of the ends of the enclosure 1.

In this example, the enclosure 1 has a substance feed inlet 4, which inlet 4 is arranged through the cover of the enclosure 1 substantially at a first end of the enclosure 1.

Naturally, the bottom and the cover of the enclosure 1 are defined relative to the ground on which the enclosure 1 is standing.

In a particular embodiment, the device has an inlet chimney 5 that is connected to the inlet 4 of the enclosure.

Preferably, the inlet chimney 5 comprises leaktight connection means 2 for connection to the inlet 4 of the enclosure 1 so as to limit the air entering the enclosure 1, which air would reduce the methane content of the gas exiting the enclosure, which is not desired. These leaktight connection means 2 also make it possible to control the flow rate of substance into the enclosure 1. By way of example, said leaktight connection means 2 comprise an airlock arranged between the inlet chimney 5 and the inlet 4 of the enclosure 1 and controlled by valves.

By way of example, the inlet chimney 5 is connected to a feed hopper or also to a unit for grinding, compacting, or granulating the substance in the form of divided solids or also to a unit for preconditioning divided solids, a preconditioning unit serving to heat and/or to dry the divided solids to specified values of temperature and of relative humidity, or indeed to densify the substance.

The enclosure 1 also includes a bottom outlet 6 that is arranged in this example in the bottom of the enclosure 1 substantially at the second of the two ends of the enclosure 1. In a particular embodiment, the device has an outlet chimney 7 that is connected in leaktight manner to the bottom outlet 6 of the enclosure 1.

Preferably, the outlet chimney 7 comprises leaktight connection means 8 for connection to the bottom outlet 6 of the enclosure 1 so as to limit the air entering the enclosure 1, which air would reduce the methane content of the gas exiting the enclosure 1, which is not desired. These leaktight connection means 8 also make it possible to control the rate at which residues are removed from the substance heat-treated in the enclosure 1. By way of example, said leaktight connection means 8 comprise an airlock that is arranged between the outlet chimney and the bottom outlet, and that is controlled by valves.

By way of example, the outlet chimney 7 is connected to a cooling unit 9 for cooling the residues, either with the aim of destroying the residues, or with the aim of recycling said residues, which could, for example, be used as fuel, possibly after one or more additional treatment steps.

In addition, the device has means for conveying the substance between the inlet of the enclosure and the bottom outlet of the enclosure. These means thus comprise a screw 10 that extends inside the enclosure 1 along an axis X between the two equipment boxes 3 and that is mounted to rotate about said axis X inside the enclosure 1. By way of example, the screw 10 may be made of stainless steel. The screw 10 thus withstands high temperatures typically lying in the range 700 degrees Celsius (° C.) to 1200° C. Specifically, the screw 10 is in the form of a helical coil that is fastened end-on at each of its two ends to respective shaft segments; the fastening being by welding, for example. Each of said shaft segments is connected at its other end via a respective flange to a shaft on the same axis that passes through the associated end equipment box.

The conveyor means also include means for driving the screw 10 to rotate about the axis X, which means are arranged in one of the equipment boxes 3. According to a particular aspect of the invention, the rotary drive means comprise an electric motor 14 and mechanical connection means between the outlet shaft of the motor and one end of the associated same-axis shaft, the shaft itself driving the screw 10. These rotary drive means in this example include control means for controlling the speed of rotation of the outlet shaft of the motor, which means may for example comprise a variable speed controller. The control means thus enable the speed of rotation of the screw 10 to be adapted to the substance it is conveying, i.e. they enable the transit time of the substance through the enclosure 1 to be varied.

The control device also includes means for heating the screw 10 by the Joule effect, which heater means are arranged in the equipment boxes 3 in this example. In a particular embodiment, the heater means comprise generator means for generating electricity and means for connecting the two ends of the screw to the two polarities of said generator means. For this purpose, each same-axis shaft is securely connected to a coaxial drum of electrically conductive material with carbon brushes rubbing thereagainst to deliver electricity, which brushes are connected by conductor wires (not shown) to the generator means for generating electricity. The screw 10 thus carries the same electric current all along the axis X.

Preferably, the screw 10 is shaped to have electrical resistance that varies along its axis X and thus makes it possible to offer different heating zones simultaneously along its axis X. In particular manner, the screw 10 is thus shaped so as to have a temperature profile such that the temperature at the inlet 4 of the enclosure 1 is greater than the temperature at the outlet 6, 11 of the enclosure 1. This makes it possible to limit the adherence of divided solids made of plastics material on the turns of the screw 10, when they enter the enclosure 1, due to melting of said divided solids under the action of heating of the screw 10.

In a particular aspect of the invention, the heater means comprise means for regulating the electric current conveyed by the screw 10. In this example the regulator means comprise a power controller interposed between the electricity generator means and the connection means. The regulator means thus enable the electric current carried by the screw 10 to be adapted to the substance being conveyed.

In this embodiment, the enclosure 1, the conveyor means, and the supply means thus form a pyrolysis reactor for the substance introduced into the enclosure 1.

In addition, the enclosure 1 also includes a top outlet 11 for the extraction of gas coming from pyrolysis of the substance, said top outlet 11 being arranged in this embodiment in the cover of the enclosure 1 substantially at the second of the two ends of the enclosure 1. In this embodiment the top outlet 11 is slightly upstream of the bottom outlet of the enclosure 1 relative to the inlet 4 of the enclosure.

The device further comprises an impurity elimination unit 12 for eliminating impurities present in the gas resulting from pyrolysis of the substance. Said unit 12 is connected to the top outlet 11 in such a manner that the gas is extracted continuously from the enclosure 1 (unlike the inlet 4 and the bottom outlet 6 which are shaped so that supply of substance and removal of residues can be done in discontinuous manner).

In a particular aspect of the invention, the impurity elimination unit 12 comprises cracking means for cracking the gas, which means are directly connected to the top outlet 11 of the enclosure 1 in this embodiment.

These cracking means make it possible to crack the tar and oil phases present in the gas so as to recover gas that is cleaner at the outlet of the cracking means.

By way of example, the cracking means comprise a cracking furnace 13 comprising a vertical tubular enclosure that comprises an inlet 15 for introducing gas and an outlet 16 for discharging said gas from the housing 14, means for heating said gas that comprise a heater tube 17 extending vertically inside the housing and coaxially with the housing, the heater tube 17 being shaped so as to have its bottom end closed and being arranged so that its bottom end is arranged in the housing 14 and so that its top end is connected to a burner 18 of the heater means, which burner is arranged outside the housing 14. Typically, the heater means further comprise an inlet pipe for a heating fuel (natural gas, fuel, purified synthesis gas, or also gas treated by the present cracking furnace 13, with a fraction of the gas being taken off at the outlet 16 of the cracking furnace 13 in order to feed the inlet pipe . . . ), which pipe is connected to a burner 18 of said heater means. The heater means also comprise an outlet pipe for burnt fuel.

Thus, the particular arrangement of the housing 14 and of the heater tube 17 makes it possible to create a treatment zone in which the gas is well confined. This makes it possible to promote heat exchange within the housing 14 between the gas for treatment and the heater tube 17: the cracking of the tar and oil phases is thus efficient and fast, in such a manner that the gas recovered at the outlet of the cracking furnace 13 presents high purity.

In this embodiment, the cracking furnace 13 is arranged in such a manner that the gas is heated to about 1100° C. in order to further improve cracking.

Preferably, the heater means make use firstly of an external heating fuel outside the cracking furnace 13 in order to initiate heating of the heater tube 17 (which fuel is of the natural gas, fuel oil, or purified synthesis gas type), and once treatment of the gas has commenced, the heater means take a fraction of the treated gas from the outlet 16 of the cracking furnace 13 in order to heat the heater tube 17.

Thus, the cracking furnace 13 is relatively independent and requires an external fuel only for initializing the start of cracking.

The external fuel could also be used in operation, when merely taking treated gas from the outlet 16 of the cracking furnace 13 is not sufficient for powering the burner 18.

Preferably, the inlet 15 extends substantially tangentially to the housing 14. The inlet 15 is thus arranged so as to cause the gas to penetrate into the housing 15 along the inside wall of the housing 15. This makes it possible to generate a cyclone effect so that the gas flows helically in the housing 15. This promotes treatment of the gas.

The elimination unit 12 further comprises filter means 19, e.g. directly connected to the outlet of the cracking furnace 13, in order to eliminate dust and solid particles still present in the gas. The filter means 19 typically comprise a high-temperature cyclone and/or a high-temperature filter (such as a ceramic filter) arranged across the duct connected to the outlet of the cracking furnace 13. The cyclone and/or the filter thus being resistant to high temperatures typically in the range 600° C. to 1000° C.

Preferably, the elimination unit 12 also comprises cooling means 20 of the heat exchanger type directly connected to the filter means 19. This makes it possible to cool the gas that has been heated during the cracking step before the gas is purified. By way of example, the gas at the inlet of the cooling means is at a temperature lying in the range 600° C. to 800° C. By way of example, the gas is cooled to approximately 40° C.

In addition, the device includes a purification system 21 for purifying the gas at the outlet of the elimination unit 12. In this example, the purification system 21 is directly connected to the outlet of the cooling means 20.

In a particular aspect of the invention, the purification system 21 includes three gas purification stages.

Preferably, the first stage comprises means for condensing condensable phases of the gas at a pressure that is less than atmospheric pressure measured at sea level (which is substantially 1 bar). Typically, the condensation means are shaped so as to ensure separation of the gas from the condensable phases at a pressure lying in the range 0.04 bar to 0.3 bar and preferably at a pressure of 0.14 bar.

To this end, the condensation means comprise a compressor 22 compressing the gas to the desired pressure, in this example 0.14 bar, the compressor additionally being connected to the outlet of the cooling means 20. The condensation means further comprise a condenser 23 that is itself directly connected to the outlet of the compressor 22 and that enables the gas to be separated from the condensable phases at 0.14 bar. In practice, the majority of said condensable phases is water. The methane content of the gas at the outlet of the first stage is thus increased.

In particular manner, the second stage comprises pressure swing adsorption equipment 24 that is connected directly to the outlet of the first stage.

Such a device is well known from the prior art and is not described in more detail below.

This makes it possible to remove almost entirely not only the carbon dioxide but also the dinitrogen that is still present in the gas, thus making it possible to further increase the methane content of the gas at the outlet of the second stage.

Preferably, the first stage and the second stage are interconnected. To this end, the first stage comprises a mixer 25 arranged upstream from the compressor 22 and connected directly firstly to the outlet of the cooling means 20 and secondly to the compressor 22. In addition, the pressure swing adsorption equipment 24 is connected to the mixer 25.

The mixer 25 thus makes it possible to mix the gas at the outlet of the elimination unit 12 with a portion of the gas being purified by the second stage in order to promote said purification and thus to transmit to the third stage a gas that has a higher methane content.

In particular manner, the third stage comprises a filtering machine 26 that is a membrane separation machine. Such a device 26 is well known from the prior art and is not described in more detail below.

In this embodiment, said machine 26 is connected directly to the outlet of the second stage.

This enables almost all of the dihydrogen still present in the gas to be removed, thus enabling the methane content to be increased even further at the outlet of the third stage (referenced G) and thus at the outlet of the purification system.

An embodiment of the device is described below.

Initially, the substance to be treated is introduced into the inlet chimney 5 in the form of divided solids and the screw 10 pushes the divided solids in continuous manner towards the bottom outlet 6 of the enclosure 1. Because of the temperature of the screw 10, the divided solids soften progressively until they melt, which generates gas that is already rich in methane.

The screw 10 thus serves both to apply heat treatment to the substance and to convey the substance.

Preferably, the substance is subjected to heat treatment at a high temperature inside the enclosure 1, typically in the range 500° C. to 1000° C. and preferably in the range 600° C. to 800° C.

Preferably, the device is shaped so that the substance remains for 10 minutes to 30 minutes inside the enclosure and even more preferably for 15 minutes to 20 minutes.

This therefore makes it possible to pyrolyze the substance effectively and thus to recover at the top outlet 11 of the enclosure 1 a gas that is already very rich in methane. Typically, the gas at the outlet of the enclosure 1 presents a methane content that is greater than 60%.

Carbon residue from the substance is thus removed at the bottom outlet 6.

In addition, the gas extracted at the top outlet 11 of the enclosure 1 passes through the impurity elimination unit 12 which makes it possible to remove successively tar and oil and then dust and particles. A cleaner gas is thus recovered at the outlet of said elimination unit 12.

The gas then passes through the purification system 21, which makes it possible to remove in succession water: carbon dioxide, dinitrogen, and dihydrogen. Thus, at the outlet of the device, a gas G is recovered that is purer in terms of methane content.

The gas G at the outlet of the purification system, and therefore of the device, thus has a very high methane content. Typically, the gas G at the outlet of the device presents a methane content that is greater than 90%.

Naturally, the invention is not limited to the embodiment described and variant embodiments may be provided without going beyond the ambit of the invention as defined by the claims.

In particular, although in this example the substance being supplied to the device is made up of plastics comprising for the most part polyethylene and polyethylene terephthalate, the device may use some other type of substance for methane production. The substance could thus be a biomass or also a polymer solid, for example such as waste formed of plastics, rubber, or elastomer, or also a solid including cardboard, a metal material such as aluminum, . . . or also a solid recovered fuel. It should be recalled that the term “biomass” designates the biodegradable fractions of substances, waste, and residues coming from industry in general and from agriculture, from sylviculture, and from associated industries, in particular.

The substance may comprise a single type of solid (polymer, plastics, SRF, biomass . . . ) or several types of solid. The divided solids may be in the form of three-dimensional granules or else they are two-dimensional flakes. In general, the divided solids may be in the form of powder, granules, pieces, fibers, . . . .

In addition, the enclosure and the associated means for conveying and heating by the Joule effect may be different from those described. By way of example, the means for leaktight connection of the feed inlet and/or of the bottom outlet may comprise elements other than the airlock, such as for example, a valve-lock or indeed a metering device. The screw and the associated means for heating by the Joule effect may thus be shaped to authorize heating of the substance in stages, the screw for example presenting electrical resistance that varies along the length of its axis and thus making it possible to provide different heating zones simultaneously along its axis.

In addition, the impurity elimination unit may comprise means other than those indicated. In particular, in place of the cracking means, said unit may comprise condensation means for condensing polluting condensates of the gas such as tar and oils, e.g. by means of a neutralizing absorber. Said unit may thus be free from cooling means.

In addition, the purification system may be different from the system described. By way of example, said system may comprise a number of purification stages other than that described. The purification system may thus comprise means for cooling the gas for treatment so as to condense said gas. The liquids are then separated before the methane gas returns to form, which methane gas is thus enriched as a result of separating the different liquids.

The device may be shaped so that the enclosure is filled with an inert gas in order to limit or eliminate the presence of oxygen inside the enclosure. 

1. A device for producing a methane gas by heat-treating a substance in the form of divided solids, the device comprising: an enclosure comprising a substance feed inlet, a bottom outlet for recovering residues of the treated substance, and a top outlet for extracting the gas coming from the treatment of the substance; conveyor means for conveying the substance between an inlet of the enclosure and the bottom outlet, the means comprising a screw mounted to rotate inside the enclosure about an axis of rotation and having drive means for driving the screw in rotation; heater means for heating the screw by the Joule effect; an impurity elimination unit for eliminating impurities present in the gas coming from the heat treatment substance, said unit being connected to the top outlet of the enclosure; and a purification system for purifying the gas at the outlet of the elimination unit, the purification system being connected to the elimination unit.
 2. The device according to claim 1, including an inlet chimney that is connected to the inlet of the enclosure and that comprises leaktight connection means for connection to the inlet of the enclosure so as to limit the air entering the enclosure.
 3. The device according to claim 1, including an outlet chimney that is connected to the bottom outlet of the enclosure and that comprises leaktight connection means for connection to the bottom outlet of the enclosure so as to limit the air entering the enclosure.
 4. The device according to claim 1, wherein the impurity elimination unit comprises cracking means for cracking the gas.
 5. The device according to claim 1, wherein the impurity elimination unit comprises condensation means for condensing polluting condensates of the gas.
 6. The device according to claim 1, wherein the impurity elimination unit comprises filter means for filtering dust and solid particles present in the gas.
 7. The device according to claim 4, wherein the filter means are connected to the outlet of the cracking means or to the outlet of the condensation means.
 8. The device according to claim 6, wherein the filter means comprise a high-temperature cyclone or a high-temperature ceramic filter.
 9. The device according to claim 1, the purification system comprises at least two distinct purification stages.
 10. The device according to claim 9, wherein the purification system comprises at least three purification stages.
 11. The device according to claim 1, wherein the purification system comprises condensation means for condensing condensable phases of the gas at a pressure that is less than atmospheric pressure measured at sea level.
 12. The device according to claim 11, wherein the condensation means are shaped so as to ensure separation of the gas from the condensable phases at a pressure lying in the range 0.04 bar to 0.3 bar.
 13. The device according to claim 1, wherein the purification system comprises pressure swing adsorption equipment.
 14. The device according to claim 11, wherein the pressure swing adsorption equipment is connected to the outlet of the condensation means.
 15. The device according to claim 1, wherein the purification system comprises a filtering machine.
 16. The device according to claim 15, wherein the filtering machine is a membrane separation machine.
 17. The device according to claim 15, wherein the filtering machine is connected to the outlet of the pressure swing adsorption equipment.
 18. An application of the device according to claim 1, wherein the substance feeding the enclosure is of the SRF type or of the polymer material type.
 19. A method of producing a methane gas by heat-treating a substance in the form of divided solids by means of a device according to claim 1, wherein the enclosure is supplied with substances made of SRF or polymer material. 